This invention relates to the magnetron sputtering, and more particularly, to magnetron magnet design for the efficient utilization of a sputtering target.
Magnetron sputtering, or magnetically enhanced sputtering, involves the use of a sputtering target to provide coating material for vapor phase deposition in a vacuum onto substrates in a chamber. In sputtering, the sputtering target is energized with a negative potential to act as a cathode in a glow discharge system. In magnetron sputtering, magnets generate a magnetic field in the form of a closed loop magnetic tunnel over the surface of the sputtering target. The magnetic tunnel confines electrons near the surface of the target. The electron confinement allows the formation of a plasma with significantly lower ignition and extinction voltages for a given process pressure, and significantly lower ignition and extinction pressures for a given cathode voltage.
A conventional magnetic tunnel confines near the target surface both thermal electrons and secondary electrons. Thermal electrons are distinguished from secondary electrons by their origins, which give the electrons different characteristics. Thermal electrons are created through an ionizing collision of an atom or ion with another electron. Thermal electrons are much more numerous than secondary electrons but have much lower energy. Secondary electrons are emitted from the target upon impact of the target by an ion.
The tunnel forces the secondary electrons into semicircular orbits along the length of the magnetic tunnel due to the influence of the component of the magnetic field that is parallel to the uneroded target surface in the cross-sectional plane. The component of the magnetic field perpendicular to the uneroded target surface forces secondary electrons moving parallel to the axis of the tunnel to move laterally in the cross-sectional plane toward the magnetic centerline of the tunnel, which is the line on the surface of the target where the magnetic field perpendicular to the uneroded target surface is zero. Thermal electrons, on the other hand, move back and forth in the cross-sectional plane of the tunnel and form helical orbits along the lines of magnetic flux. Because of their low mobility perpendicular to the lines of magnetic flux, thermal electrons are confined to the region of the cathode. Magnetic mirrors created by converging magnetic flux lines at the edge of the magnetic tunnel keeps the thermal electrons reflecting from side to side as the low mobility of the thermal electrons perpendicular to the magnetic flux lines keeps them moving in helical orbits around the flux lines. The mirror effect is strongest at the edges of the magnetic tunnel and vanishes above the magnetic centerline of the tunnel, so that the thermal electrons move horizontally in a one-dimensional potential well that is centered at the magnetic centerline of the tunnel.
The electron confining properties of magnetron sputtering are effective in enhancing plasma density near the target surface and make magnetically enhanced sputtering much more practical than traditional diode sputtering. However, the conventional magnetic tunnel is responsible for low target utilization because the concentration of electrons near the magnetic centerline of the tunnel causes the plasma density to be similarly concentrated, making the erosion rate highest in this region. Further, as the target erodes, the target surface in the area adjacent the centerline erodes into even stronger magnetic fields, which accelerates the concentration of the erosion. In addition, the concentrated erosion produces a V-shaped profile redirecting secondary electrons from the opposite walls to the center of the erosion groove, further concentrating the plasma there. Typically, utilization of a target of uniform thickness has been approximately 25%. Poor target utilization undermines the economics of thin film deposition by increasing the number of expended targets and amount of unused target material as well as the machine downtime required for target changes. Uneroded areas of the target tend to occur near the edges of the magnetic tunnel. Where such areas exist, they tend to accumulate redeposited material that flakes off into the processing chamber to produce particulate contamination of the substrate.
Magnetic tunnels have been employed having nonsymmetrical shapes that are rotated with respect to the target to manipulate the erosion profile. Such rotation is useful in achieving improved film uniformity on the substrate, achieving higher target utilization and eroding points on the target that would otherwise be left uneroded if the magnetic tunnel were static. These rotating arrangements are only convenient for round planar targets. For rectangular and annular targets, only static magnetic arrangements have been practical.
For frusto-conical targets and other annular or ring-shaped targets having other system components located in the center of the target, the magnetron arrangements of the prior art have not provided high target utilization. While magnetically enhanced sputtering has made sputtering a practical and economically viable technique for depositing thin films, its full economic potential has not been realized in the case of static magnetic arrangements.
Accordingly, there remains a need to provide a magnet design that will produce high target utilization with frusto-conical and other annular targets.
An objective of the present invention is to provide for improved utilization and full face erosion of a frusto-conical target in a magnetron sputtering apparatus. A particular objective of the present invention is to provide for improved utilization of an annular target without restricting availability of the volume within the opening in the center of the target to use for other hardware, such as, for example, an ICP source.
In accordance with the principles of the present invention, a sputtering apparatus that includes an frusto-conical or similar annular sputtering target is provided with a magnetron magnet assembly that causes the erosion of the target to move from the center of the target annulus to inner and outer regions as the target erodes. The magnet assembly is positioned behind the target to produce a plasma confining magnetic field over the target in the shape of an annular tunnel on the surface of the annular target surrounding the opening at the target center. The walls of the target form a truncated cone that is inclined, for example, at about 35xc2x0 to the plane of the central opening.
In one embodiment of the invention, three permanent magnet rings produce three magnetic tunnels. The relative contributions of the three tunnels produces the effect of magnetic flux lines that are parallel to the surface of the target. For a thin target, and as some point part way through the lives of thicker targets, an inner magnetic tunnel and an outer magnetic interact with a main central magnetic tunnel to produce a resultant magnetic flux parallel to the surface of the uneroded target. With such thicker targets, a main central tunnel dominates early in the target""s life to erode the mean radius of the annular target along the target centerline and inner and outer tunnels dominate later in the life of the target to erode areas adjacent the inner and outer rims of the target annulus and spread the erosion groove inward and outward of the target centerline in the same manner as if the flux remained parallel to the uneroded target surface throughout the life of the target.
In the preferred embodiment, a sputtering apparatus, is an ionized physical vapor deposition apparatus that includes a vacuum processing chamber, a substrate support in the processing chamber for supporting a substrate for processing, an annular magnetron sputtering cathode assembly having a central opening and an inductively coupled plasma source behind a dielectric window in the central opening. The magnetron cathode assembly includes a frusto-conical sputtering target having an interior conical sputtering surface facing the substrate support with the outer edge of the target closer to the substrate support than the inner edge. A frusto-conical magnet assembly is situated behind, and parallel to, the sputtering target. The magnet assembly is configured to produce a main magnetic tunnel having magnetic field lines spanning a major portion of the sputtering surface and straddling the centerline of the sputtering surface, an inner magnetic tunnel having magnetic field lines extending between the target inner edge and centerline, and an outer magnetic tunnel having magnetic field lines extending predominantly between the outer edge and centerline of the target. The magnetic fields of the three tunnels interact in a way that tends to produce a resulting magnetic flux that is relatively parallel to the target surface. For targets that are thicker, this resultant flux tends to arc over the target centerline early in the target""s life, become flatter part way into the target""s life and gradually and progressively take on the shape of two tunnels, one inside of the target centerline and one outside of the target centerline. In this way, where erosion of the target proceeds at a greater rate at the centerline at the beginning of the target""s life, compensating erosion will occur toward the inner and outer edges of the target later in the target""s life so that target utilization is uniform over the entire life of the target.
The relative strengths of the magnetic fields are such that plasma confined by the magnetic flux that is shaped thereby produces a greater target erosion rate at the centerline than at the inner and outer areas thereof when the target is uneroded which continuously and progressively changes to a lesser target erosion rate at the centerline than at the inner and outer areas thereof as the target erodes. This effect is greater for thicker targets. At some point in the life of the target, the magnetic flux is parallel to that which was the surface of the target before the target eroded. For very thin targets, this flux shape may exist throughout the target""s life. For thin as well as thick targets, the flat shape of the magnetic flux or a progressive flattening of the magnetic flux are the result of the relative contributions of the magnetic rings that produce the three magnetic tunnels.
Preferably, the configuration and strengths of the magnetic rings are optimized for specific targets. For example, where a target is very thin, the relative contributions of the main tunnel and the inner and outer tunnels are adjusted such that the resultant magnetic flux lines are parallel to the target surface. For thicker targets, the relative contributions of the main tunnel and the inner and outer tunnels are adjusted such that the resultant magnetic flux lines form a tunnel at the target surface that is similar to that of a conventional magnetic arrangement at the beginning of target life, but as the target erodes the shape of the flux lines becomes flat so as not to pinch the plasma. As the target erodes further, the inner and outer tunnels are exposed to the plasma as the target surface recedes into these tunnels, resulting in higher plasma density near the inner and outer edges of the annular conical target.
The frusto-conical magnet assembly preferably includes an inner pole of a first polarity and an outer pole of a second polarity producing the first magnetic tunnel by a first magnetic field extending between the inner and outer poles. The magnet assembly preferably further includes an inner-central pole of the second polarity and an outer-central pole of the first polarity producing a reverse magnetic field opposing the first magnetic field on the centerline of the target such that the resulting strength of the first and reverse magnetic fields decreases on the centerline as the sputtering surface erodes into the target. Preferably, the inner pole and the inner-central pole produce the inner magnetic tunnel formed of magnetic field lines extending between the inner and inner-central poles that lie beneath the main magnetic tunnel over the annular inner area, while the outer pole and the outer-central pole produce the outer magnetic tunnel formed of magnetic field lines extending between the outer and outer-central poles beneath the main magnetic tunnel over the annular outer area.
The preferred magnet assembly is formed of permanent magnets in the form of a plurality of preferably three circular magnetic rings arranged in a cone behind, and parallel to, the conical sputtering target. The rings are preferably made up of individual square magnets arranged in a frusto-conical shape, with their polar axes extending perpendicular to the circumference of the rings. The three rings include an inner, an outer and an intermediate magnetic ring. The polar axes of the magnets of the inner and intermediate rings are oriented parallel to the cone and those of the outer ring are perpendicular to the cone. A yoke of magnetically permeable material lies behind the cone and magnetically interconnects the inner and outer magnetic rings.
The present invention improves the utilization of frusto-conical targets from previously about 25% into the range of 50-60%. Erosion of a conical target is maintained over the entire area of the sputtering surface of the target, thereby avoiding the production of particulate contamination of the sputtering chamber.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description.